What does the box keyword do?

Question

In Rust, we can use the Box type to allocate things on the heap. This type is used to safely abstract pointers to heap memory. Box

Answer 1

box does exactly what Box::new() does - it creates an owned box.

I believe that you can't find implementation of box keyword because currently it is hardcoded to work with owned boxes, and Box type is a lang item:

#[lang = "owned_box"]
#[stable(feature = "rust1", since = "1.0.0")]
#[fundamental]
pub struct Box(Unique);

Because it is a lang item, the compiler has special logic to handle its instantiation which it can link with box keyword.

I believe that the compiler delegates box allocation to functions in alloc::heap module.

As for what box keyword does and supposed to do in general, Shepmaster's answer describes perfectly.